The present invention relates to a cluster ion beam (CIB) method for forming thin oxidation layers in devices used for data storage and retrieval or any application in which detection of small magnetic fields is the method of operation. For example, the CIB method is applicable for forming specular reflecting layers in spin valve sensors for increasing the giant magnetoresistive ratio of the magnetic element, or for forming tunnel barrier layers in tunnel magnetoresistive devices.
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Magnetic heads, including read sensors, are then used to read data from the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors (MR sensors) are the prevailing read sensors. An MR sensor detects a magnetic field through the change in resistance of its MR sensing layer (MR element) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
One type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect, and another type is a tunnel magnetoresistance (TMR) sensor manifesting the TMR effect. In GMR sensors, the resistance of the MR element varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a nonmagnetic, conductive layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers. In TMR sensors, the resistance of the MR element varies as a function of the tunneling current allowed to pass between magnetic layers through a nonmagnetic, insulating layer (barrier layer).
GMR sensors using two layers of ferromagnetic material separated by a layer of nonmagnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect. In a spin valve sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, is not fixed and is free to rotate in response to the field from the recorded magnetic medium. In spin valve sensors, the spin valve effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of the magnetization in the free layer, which in turn causes a change in resistance of the spin valve sensor and a corresponding change in the sensed current or voltage.
FIG. 1 shows a typical simple spin valve 10 (not drawn to scale) comprising a central region 12 separating end regions 14 formed on a substrate 16. In central region 12, a free layer (free ferromagnetic layer) 18 is separated from a pinned layer 20 (pinned ferromagnetic layer) by a nonmagnetic, electrically-conducting spacer layer 22. The magnetization of the pinned layer 20 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 24. FIG. 1 is an air bearing surface (ABS) view, and the arrows indicate that the free layer 18 has a magnetization direction, in the absence of an external magnetic field, parallel to the ABS and the pinned layer 20 has a magnetization direction perpendicular or 90xc2x0 to the ABS, wherein the ABS is an exposed surface of the sensor that faces the magnetic medium. Hard biased layers 26 are formed in the end regions 14 to provide longitudinal bias for the free layer 18. Leads 28 are formed over hard biased layers 26 and provide electrical connections for the flow of a sensing current from a current source 30 to the sensor 10. Sensor device 32 is connected to leads 28 and senses the change in the resistance due to the changes induced in the free layer 18 by the external magnetic field. The construction depicted in FIG. 1 is the simplest construction for a spin valve sensor, and is well known in the art.
Another type of spin valve sensor is an antiparallel (AP) pinned spin valve sensor. In this type of magnetic element, a laminated AP pinned layer structure is substituted for the single pinned layer in FIG. 1. The AP pinned layer structure includes a nonmagnetic AP coupling layer (APC layer) between first and second AP pinned layers (AP1 and AP2, respectively). The AP1 pinned layer is exchange coupled to the antiferromagnetic pinning layer, which pins the magnetic moment (magnetization direction) of the AP1 pinned layer in the same direction as the magnetic spins of the pinning layer. By exchange coupling between the AP1 and AP2 layers, the magnetic moment of the AP2 pinned layer is pinned antiparallel to the magnetic moment of the AP1 pinned layer. An advantage of the AP pinned layer structure is that demagnetization fields of the AP1 and AP2 pinned layers partially counterbalance one another so that a small demagnetization field is exerted on the free layer for improved biasing of the free layer.
FIG. 2 shows an exemplary AP pinned spin valve sensor 10xe2x80x2 (not drawn to scale) of the prior art. As with sensor 10 of FIG. 1, spin valve sensor 10xe2x80x2 has a central region 12 separating end regions 14 formed on substrate 16. AP pinned spin valve sensor 10xe2x80x2 comprises free layer 18 separated from a laminated AP pinned layer structure 40 by spacer layer 22. The magnetization of the laminated AP pinned layer structure 40 is fixed by the AFM pinning layer 24. The laminated AP pinned layer structure 40 includes a first ferromagnetic layer (AP1 layer) 42 and a second ferromagnetic layer (AP2 layer) 44 separated from each other by an antiparallel coupling layer (APC layer) 46. As with sensor 10 in FIG. 1, hard bias layers 26 are formed in end regions 14 to provide longitudinal biasing for the free layer 18, and electrical leads 28 provide electrical current from current source 30 to the spin valve sensor 10xe2x80x2. Sensor device 32 is connected to leads 28 to sense the change in resistance due to changes induced in the free layer 18.
Various parameters of a spin valve sensor may be used to evaluate the performance thereof. Examples of such parameters include the structure sheet resistance (R) and the GMR ratio (xcex94R/R), also referred to as the GMR coefficient. The GMR ratio is defined as (RAP-RP)/RP, where RAP is the antiparallel resistance and RP is the parallel resistance. The GMR ratio is an expression of the magnitude of the sensor response, and thus, the operation of a spin valve sensor is maximized by maximizing the GMR ratio. The GMR effect depends on the angle between the magnetizations of the free and pinned layers. In a spin valve sensor, the electron scattering, and therefore the resistance, is maximum when the magnetizations of the pinned and free layers are antiparallel, i.e., a majority of the electrons are scattered as they try to cross the boundary between the MR layers. On the other hand, electron scattering and therefore the resistance is minimum when the magnetizations of the pinned and free layers are parallel, i.e., a majority of electrons are not scattered as they try to cross the boundary between the MR layers. Thus, there is net change in resistance of a spin valve sensor between parallel and antiparallel magnetization orientations of the pinned and free layers. The GMR effect, i.e., the net change in resistance, exhibited by a typical prior art spin valve sensor, such as that shown in FIG. 2, is about 6% to 8%.
The disk drive industry has been engaged in an ongoing effort to increase the overall sensitivity, or GMR ratio, of the spin valve sensors to permit the drive head to read smaller changes in magnetic flux. Higher GMR ratios enable the storage of more bits of information on any given disk surface, and ultimately provide for higher capacity disk drives without a corresponding increase in the size or complexity of the disk drives.
It is well known that the addition of specular reflecting layers increases the GMR ratio of spin valve films. The GMR ratio is highly dependent upon the specular scattering that occurs within the pinned layer and the free layer of the sensor, with higher specular scattering resulting in a higher GMR ratio. Specular reflectors may be formed of materials similar to the ferromagnetic material forming the pinned layer or the free layer. For example, oxides of cobalt, iron and nickel, or a mixture of these oxides, are suitable as specular reflecting materials. These specular reflectors may be formed by oxidizing the ferromagnetic film, or by sputtering onto the ferromagnetic film using an oxide target. The oxidized metal layers may also be referred to as nano-oxide layers (NOLs). Oxide layers are ideal reflectors due to their electronic properties.
The specular layers can be added to the free layer structure (free layer NOL 50), as shown in FIG. 2, as well as to the pinned layer structure (AP NOL or bottom NOL)(not shown). The AP NOL is particularly challenging because it is located in the middle of the pinned layer in a spin valve stack and therefore may affect the pinning strength and the growth of the layers on top of it. Bottom NOL spin valves formed by oxidizing CoFe may have enhanced GMR properties, but typically have degraded pinning strength and poor thermal stability, which has prevented the use of bottom NOL spin valves in production processes.
There is therefore a need to develop a method for forming an AP NOL layer and other specular reflecting layers in a spin valve thin-film magnetic element in which the GMR ratio is increased by the use of an AP NOL layer without degradation in pinning strength and thermal stability.
In other devices, such as a TMR device, either for magnetic recording heads or MRAM applications, wherein an oxide layer is formed between magnetic layers, natural oxidation is often used for oxidation of metal layers, which produces inconsistent results due to the low surface activation and low energy of the oxygen atoms. The oxide layer may also contribute to low thermal stability wherein the magnetic properties of the device are degraded after exposure to high temperature annealing. Because the device performance is critically dependant on the quality of this oxide layer, a repeatable and uniform method of oxidation is desired. The uniformity of the oxide relates to the device yield across a wafer. It is very challenging to produce a uniform oxide layer across a large wafer.
Thus, there is a need in any magnetic element application having an oxide/magnetic layer interface for an oxidation method that increases the bonding strength at the interface, increases the thermal stability of the device, and increases the consistency and repeatability of the oxidation results for production.
The present invention provides a method of forming a thin-film magnetic element on a substrate wherein at least a surface portion of a nonmagnetic metal layer is oxidized by cluster ion beam (CIB) oxidation. Specifically, the method comprises depositing a first magnetic layer on a substrate, then depositing a nonmagnetic metal layer on the first magnetic layer. At least a top surface of the nonmagnetic layer is then oxidized by CIB oxidation. In one embodiment, only a top surface portion is oxidized such that a nano-oxide layer (NOL) is formed from the nonmagnetic conductive layer. For example, the nonmagnetic metal layer may be a ruthenium APC layer in a bottom pinned structure of a spin valve, whereby an AP NOL layer comprising RuOx is formed by CIB oxidation of the top surface portion of the ruthenium. In another embodiment, the nonmagnetic metal layer is oxidized throughout its thickness such that the layer is converted to a nonmagnetic insulating film. For example, the nonmagnetic metal layer may be an aluminum layer for forming an Al2O3 tunnel barrier layer in a TMR device. After CIB oxidation, the method further comprises depositing a second magnetic layer on the oxidized layer whereby improved adhesion is achieved at the interface thereof.
In the method of the present invention, oxidizing by cluster ion beam oxidation advantageously comprises mixing a pressurized inert carrier gas, such as argon, with oxygen gas to form a gas mixture and passing the gas mixture into a low pressure vacuum to produce a supersonic gas jet, whereby expansion occurs in the jet to cause formation of clusters of inert gas and oxygen atoms and molecules. The clusters are then ionized to form cluster ions, which are then focused into a cluster ion beam and accelerated toward the top surface of the nonmagnetic metal layer to bombard the top surface and react the ionized oxygen molecules with at least the top surface of the nonmagnetic metal layer.